High temperature thermal barrier coating

ABSTRACT

A high temperature thermal barrier coating which consists of a stabilized ZrO 2  composition for the protection of thermally loaded components ( 10, 10 ′) of a thermal machine, especially a gas turbine, is disclosed. The thermal barrier coating is stabilized with at least 15 mol % Y 1+v  Ta 1−v O 4−v , the ZrO 2  is partially substituted by at least 10 mol % HfO 2  and the composition is established according to the formula 
       (Y 1+v Ta 1−v O 4−v ) z (Zr 1−x Hf x O 2 ) 1−z , 
     with x ranging from 0.1 to 0.5, v ranging from −0.1 to 0.2 and z ranging from 0.15 to 0.25.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to PCT/EP2012/071839 filed Nov. 5, 2012, which claims priority to Swiss application 01800/11 filed Nov. 10, 2011, both of which are hereby incorporated in their entireties.

TECHNICAL FIELD

The present invention relates to the technology of high-temperature components for thermal machines, especially for gas turbines. It refers to a high temperature thermal barrier coating according to the preamble of claim 1.

BACKGROUND

Advancements in the technology of gas turbines depend mainly on the development of advanced materials for its components, especially blades, vanes, heat shields or combustor liners. On the other hand, a growth in the gas turbine market can be made compatible with the goals of the Kyoto Protocol by increasing the efficiency of gas turbine plants and working towards CO₂ caption and storage (which will reduce the overall plant efficiency). Increase of efficiency can be achieved either by reduction of cooling for turbine blades and combustor liners or by increasing firing temperature.

In the past, it has been shown that Thermal Barrier Coatings (TBCs) of the blades or other components prepared with Yttria Stabilized Zirconia (YSZ) reached the intrinsic limits of the material. An increase of operating temperature can be obtained by either incorporating new stabilizing elements to zirconia or by using new protection materials. The goal is a new generation of gas turbines which require TBCs being able to withstand temperatures around 1400 ° C. during more than 20′000 hours. These high operating temperatures require new coating solutions.

In the prior art, an increase of the high temperature stability of zirconia by stabilizing it with two or more oxides has been proposed (see for example documents U.S. Pat. No. 7,087,266, U.S. Pat. No. 7,060,365, U.S. Pat. No. 6,960,395, U.S. Pat. No. 6,890,668 and US 2006/0046090), or entrapping stabilizing carbon or a carbon containing gas in the pores (see documents U.S. Pat. No. 6,998,172 or US 2005/0126494), or providing an alumina containing environmental barrier in the upper layer (see documents U.S. Pat. No. 7,008,674 and U.S. Pat. No. 6,893,750), or compounds having a rhombohedral phase (see documents US 2006/0121294 and US 2006/0121293) or a cubic zirconia phase (see documents U.S. Pat. No. 6,887,595 and US 2005/0170200).

Others are working on improving the thermal stability of zirconia by incorporating high amounts of ytterbia or yttria (see document U.S. Pat. No. 6,946,208 or U.S. Pat. No. 6,930,066), or on alumina containing top layer providing an environmental barrier (see document U.S. Pat. No. 6,929,852), or on new oxides having a pyrochlore structure (see documents U.S. Pat. No. 6,835,465, U.S. Pat. No. 6,387,539, U.S. Pat. No. 6,387,526, U.S. Pat. No. 6,365,281, US 2006/0286401 and US 2006/0245984).

Further work is done on increasing the stability of zirconia by mixing it with Er203 (see document U.S. Pat. No. 6,916,551), on Re_(x)Zr_(1−x),C_(y), (see documents U.S. Pat. No. 7,041,383 and U.S. Pat. No. 6,803,135). Document U.S. Pat. No. 7,001,859 relates to work on zirconia stabilized with a primary stabilizer together with two cluster-forming dopants.

Since YSZ stabilized with 6-8 weight percent of yttria decomposition kinetic becomes significant at temperatures >1200° C. it cannot operate for extended operating intervals with surface temperatures above 1200° C. There are numerous materials that can theoretically withstand surface temperatures up to 1400° C. But up to now no one has been fully proven to be suited for use in TBC, mostly because their chemical stability in combustion environment, therm-mechanical compatibility and chemical stability with other TBC constituents are only partially understood.

SUMMARY

It is an object of the present invention to provide a high-temperature thermal barrier coating (HT-TBC), especially for gas turbine blades, capable to withstand the operation at around 1400° C. during 20′000 h and having a thermal conductivity (TC) κ<2 W/mK for the temperature range 1000° C.-1400° C. of the TBC system.

It is another object of the invention to provide a component for a thermal machine being protected by such a HT-TBC.

This and other objects are obtained by a coating according to claim 1.

The high temperature thermal barrier coating for the protection of thermally loaded components of a thermal machine, especially a gas turbine, consists of a stabilized ZrO₂ composition. It is characterized in that said composition is stabilized with at least 15 mol % Y_(1+v)Ta_(1−v)O_(4−v), the ZrO₂ is partially substituted by at least 10 mol % HfO₂ and the composition is established according to the formula

(Y_(1+v)Ta_(1−v)O_(4−v))_(z)(Zr_(1−x)Hf_(x)O₂)_(1−z),

with x ranging from 0.1 to 0.5, v ranging from −0.1 to 0.2 and z ranging from 0.15 to 0.25.

Especially, x will range from 0.15 to 0.25, v will range from 0 to 0.1 and z will range from 0.18 to 0.23.

According to another embodiment of the invention the high temperature thermal barrier coating covers a component made of a base alloy.

According to another embodiment of the invention a bondcoat is provided between the base alloy of the component and the high temperature thermal barrier coating.

Especially, the bondcoat consists of MCrAlYX, where M═Ni, Co or Fe or a mixture thereof and X═Si, Ta, B, Ca or Mg or a mixture thereof, and said bondcoat is deposited by a thermal spray process.

According to another embodiment of the invention an intermediate layer is provided between the bondcoat and the high temperature thermal barrier coating.

Especially, the intermediate layer consists of 7YSZ deposited by a thermal spray process.

According to another embodiment of the invention the high temperature thermal barrier coating is deposited by a thermal spray process.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is now to be explained more closely by means of different embodiments and with reference to the attached drawings.

FIG. 1 shows X-ray diffraction curves (intensity dependant on diffraction angle 2θ) for (YTaO₄)_(0.2)(Zr_(1−x)Hf_(x))_(0.8) treated at 1500° C. for 4 h with different x-values;

FIG. 2 shows a section of a thermally protected component according to an embodiment of the invention; and

FIG. 3 shows a section of a thermally protected component according to another embodiment of the invention.

DETAILED DESCRIPTION

The invention is intended at providing a thermal barrier coating material and system thermodynamically stable at temperatures up to >1400° C. The material is a tetragonal stabilized material which is stable at high temperature and has a high toughness value. Most of the high temperature stable zirconia have a cubic structure not allowing them to have a high toughness due to the absence of ferroelasticity in the cubic system.

Thermal barrier coating systems made of 6-8 wt % Y₂O₃ stabilized ZrO₂ (7YSZ) are not suited for use at temperatures above 1300° C. due to the decomposition of the metastable tetragonal phase into cubic, and low Y₂O₃ content zirconia which transforms into monoclinic upon cooling. The monoclinic to tetragonal transformation leads to a volume change and will thus reduce the life of the coating. Fully stabilized zirconia systems have been considered as TBC material, but they have not the high toughness that has tetragonal 7YSZ, due to the absence of a ferroelastic mechanism. For this reason a stable tetragonal zirconia composition is highly desirable.

ZrO₂ stabilized with YTaO₄ has been proposed as a TBC material having a tetragonal unit cell and being stable at high temperature. The present invention proposes a ZrO₂ composition stabilized with YTaO₄ where ZrO₂ is partially substituted by HfO₂, bringing two advantages:

-   -   1. increased tetragonality leading to an increase of fracture         toughness; and     -   2. an increased density of scattering centres for phonon         diffusion, leading to a reduction of thermal conductivity.

The HT-TBC of the invention can be applied as a single layer ceramic layer directly on the bondcoat (see FIG. 2), or as a dual layer system where 7YSZ is deposited as an intermediate layer on the bondcoat and the new HT-TBC material is deposited on the 7YSZ layer (see FIG. 3). High temperature annealing of the proposed material composition shows no decomposition at 1500° C. after 4 hours of annealing, a condition where a 7YSZ system will be fully decomposed.

FIG. 2 shows a section of a component according to an embodiment of the invention, the component 10 being made of a base alloy 11, which is covered by a bondcoat 12, the bondcoat 12 being directly covered by a HT-TBC 13 according to the invention.

FIG. 3 shows a section of a component according to another embodiment of the invention, the component 10′ being made of a base alloy 11, which is covered by a bondcoat 12, the bondcoat 12 being covered by a HT-TBC 13 according to the invention with an intermediate layer 14 being provided between the bondcoat 12 and the HT-TBC 13.

An embodiment of the thermal barrier coating according to the invention comprises an MCrAlYX bondcoat (with M═Ni, Co or Fe or a mixture thereof and X═Si, Ta, B, Ca or Mg or a mixture thereof) deposited by a thermal spray process, a HT-TBC layer deposited by a thermal spray process composed of (Zr_(1−x)Hf_(x))O₂ doped with 15-25 mol % of YTaO₄, x ranging from 0.1 to 0.5, and possibly an intermediate layer of 7YSZ deposited by thermal spray separating the bondcoat from the HT-TBC layer.

The TaYSZH system has the following compositions:

-   -   1. 20 mol % YTaO₄ in (Zr_(1−x)Hf_(x))O₂ with x=10% to 25%     -   2. 20 mol % YTaO₄ in (Zr_(1−x)Hf_(x))O₂ with x=50%

Synthesis is done by reverse co-precipitation with NH₄OH as precipitating agent.

The results are:

-   -   1. Single tetragonal phase from composition 1.     -   2. Single tetragonal phase +0.48 mol % m-ZrO₂ from composition 2     -   3. Single tetragonal phase was stable at 1500° C. for 4 h     -   4. Tetragonality increases with increasing Hf content

FIG. 1 shows X-ray diffraction curves (intensity dependant on diffraction angle 2θ) for a 20 mol % YTaO₄(Zr_(1−x)Hf_(x)) treated at 1500° C. for 4 h with x-values of x=0 (curve A), x=0.1 (curve B), x=0.25 (curve C) and x=0.5 (curve D).

The general composition of the new coating material is

(Y_(1+v)Ta_(1−v)O_(4−v))_(z)(Zr_(1−x)Hf_(x)O₂)_(1−z),

with x ranging from 0.1 to 0.5, v ranging from −0.1 to 0.2 and z ranging from 0.15 to 0.25. Preferably, x will range from 0.15 to 0.25, v will range from 0 to 0.1 and z will range from 0.18 to 0.23.

The thermal barrier coating according to the invention is intended to be used for providing a thermal protection to hot parts of thermal machines, especially gas turbines, like blades, vanes, heat shields, burners or combustor liners. It gives the advantage of increasing the temperature capability of current thermal barrier coating systems by:

-   -   1. reduced decomposition kinetic compared to currently Yttria         Stabilized Zirconia YSZ, which decomposes into a cubic and         monoclinic phase upon long term exposure at high temperatures;         the proposed compositions show very little decomposition at         temperatures up to at least 1500° C., whereas current TBC are         already significantly decomposed after few hundred hours at         temperatures as low as 1200° C.     -   2. the increased tetragonality of the system brings an         improvement in fracture toughness of the material; this allows         to reduce crack growth in the ceramic upon thermal cycling. 

1. A high temperature thermal barrier coating for the protection of thermally loaded components of a thermal machine, especially a gas turbine, said thermal barrier coating comprising a stabilized ZrO₂ composition, wherein said composition is stabilized with at least 15 mol % Y_(1+v)Ta_(1−v)O_(4−v), the ZrO₂ is partially substituted by at least 10 mol % HfO₂ and the composition is established according to the formula (Y_(1+v)Ta_(1−v)O_(4−v))_(z)(Zr_(1−x)Hf_(x)O₂)_(1−z), with x ranging from 0.1 to 0.5, v ranging from −0.1 to 0.2 and z ranging from 0.15 to 0.25.
 2. The high temperature thermal barrier coating according to claim 1, wherein x will range from 0.15 to 0.25, v will range from 0 to 0.1 and z will range from 0.18 to 0.23.
 3. The high temperature thermal barrier coating according to claim 1, wherein the high temperature thermal barrier coating covers a component made of a base alloy.
 4. The high temperature thermal barrier coating according to claim 3, further comprising a bondcoat provided between the base alloy of the component and the high temperature thermal barrier coating.
 5. The high temperature thermal barrier coating according to claim 4, wherein the bondcoat consists of MCrAlYX, where M═Ni, Co or Fe or a mixture thereof and X═Si, Ta, B, Ca or Mg or a mixture thereof, and said bondcoat is deposited by a thermal spray process.
 6. The high temperature thermal barrier coating according to claim 4, further comprising an intermediate layer provided between the bondcoat and the high temperature thermal barrier coating.
 7. The high temperature thermal barrier coating according to claim 6, wherein the intermediate layer consists of 7YSZ deposited by a thermal spray process.
 8. The high temperature thermal barrier coating according claim 1, wherein the high temperature thermal barrier coating is deposited by a thermal spray process. 